Reusable launch vehicles (RLV's) such as the Space Shuttle Orbiter utilize reusable thermal protection systems (TPS's) for thermal protection during launch, orbit, and reentry into the atmosphere. The TPS must simultaneously perform as a radiator, a reflector, and an insulator in order to, respectively, emit heat from the surface of the vehicle, prevent on-orbit heating, and protect the structure of the vehicle from residual heat flux.
During a typical reentry, the outer surface of the Orbiter is subjected to heating in excess of 2300° F. The underlying structure of the Orbiter is primarily aluminum and graphite epoxy, and the TPS must protect the structure from any temperatures exceeding 350° F. Therefore, insulation used with the TPS must have extraordinarily low thermal conductivity while being able to withstand the physical rigors of space flight.
Most thermal insulation consists of reusable surface insulation tiles. The tile substrate materials and coating selections are dependent upon the mechanical and thermal requirements of the particular location on the vehicle. For example, tiles located on the upper surface of the forward fuselage experience much lower temperatures and require less strength then tiles on the nose of the vehicle. The tiles are typically low-density blocks of rigid ceramic fibers such as alumina or silica fibers and have a typical width and length of 6 inches by 6 inches.
Recently, it has become commonplace to use flexible blanket insulation, often called flexible insulation (FI), in place of ceramic tiles as a part of the TPS of RLV's. The flexible insulation is basically a layer of pliable alumina or other ceramic batting sandwiched between layers of ceramic fabric. The fabric layers and batting are sewn together with a ceramic thread to form a quilted insulation blanket. The outer portion of the blanket is coated with a ceramic slurry, which dries and forms a ceramic coating on the top fabric portion of the blanket. The typical ceramic matrix is known as a C-9™ coating, which consists of Ludox™ silica sol, available from W. R. Grace & Co., Columbia, Md., and a silicon carbide high emissivity agent. A blanket treated in such a manner can withstand multiuse temperatures of 1200° F. with a one-time use temperature of 1400° F. under static conditions.
The flexible insulation is favorable for use on RLV's because it is much easier to maintain and replace than individual insulation tiles. Large unitary pieces of flexible insulation blanket may be constructed to conform to the contours of a vehicle, which would have previously required hundreds of individually installed tiles for insulation. The flexible insulation is also able to withstand undulations and vibrations of the underlying vehicle better than the ceramic tiles, which are rigid and brittle and must be carefully spaced along an undulating vehicle surface to prevent the tiles from fracturing. Another advantage of blankets is that tile systems weigh more then blanket systems and must also have a strain isolation pad (SIP) along with insulation filler bars between tiles.
There are some drawbacks associated with the use of traditional flexible blankets. The flexible blankets have a somewhat rough, quilted upper fabric layer. The C-9™ coating used to coat most blankets is made of coarse SiC particles in a liquid silica sol medium. These coarse SiC particles do not penetrate into the fibers of the outer blanket cloth and therefore are captured on the surface of the coarse fibers. To be compatible with the C-9™ coating, the upper layer of fabric must use a coarse fabric weave. Further, because the outer layer of fabric and the batting are loosely quilted together, the outer layer of fabric does not provide a uniform substrate for forming a flat, smooth surface. For instance, during drying of the C-9™ coating, the upper fabric layer tends to buckle and to cure as a slightly irregular surface. Once cured, the coarse C-9™ coatings sometimes have a tendency to flake off of the blanket during use.
The rough and irregular surfaces of flexible insulation blankets have heretofore made them unsuitable for use on windward surfaces of vehicles or surfaces of vehicles which experience relatively high temperatures, such as the nose section and leading edges of the vehicle. When in use, the irregular surface of the blankets causes early tripping of the boundary layer as well as increased friction along the surface of the vehicle upon reentry, thereby generating large amounts of excessive heat. Therefore, flexible insulation blankets have only been used upon leeward surfaces of the RLV's which do not experience aero turbulence or high temperatures during reentry.
What is needed is a flexible insulation blanket having a smooth, uniform, durable, light weight, and thin protective ceramic layer and a method of producing such a blanket. Further, what is needed is a method of producing a flexible insulation blanket having a uniform ceramic layer without imparting unfavorable weight or insulative characteristics to the blanket.